At present, a reduction in emission of carbon dioxide, which is a greenhouse gas, into the atmosphere is urgent. In order to reduce carbon-dioxide emissions, in addition to a method for limiting the generation of carbon dioxide itself, a method for storing carbon dioxide underground has been studied.
An available method for storing carbon dioxide underground in as large an amount as one million tons per year is to inject carbon dioxide into a geological stratum. FIG. 13 shows a carbon-dioxide-storing apparatus 80. An injection well 87, which is a tubular body, is extended to a storage layer 91 where carbon dioxide is to be stored. Carbon dioxide stored in a carbon dioxide tank 81 is injected into the storage layer 91 via the injection well 87 by means of a pumping apparatus 83.
In this case, after injection of carbon dioxide into the storage layer 91, it is desirable that injected carbon dioxide does not exude above ground. Thus, as shown in FIG. 13, a seal layer 89 having an anticlinal structure (upwardly convex form) must be present above the storage layer 91. The seal layer 89 is a layer through which carbon dioxide is unlikely to penetrate; for example, an argillaceous layer.
The seal layer 89 prevents carbon dioxide injected underground from exuding above ground. However, a geological formation having such an upwardly convex seal layer 89 is present only in limited locations; i.e., locations available for application of such a method are limited.
Therefore, there has been studied a method applicable to a location where the seal layer 89 is not of an anticlinal structure, but of a monoclinal structure; specifically, a method for efficiently storing carbon dioxide in groundwater through dissolution of carbon dioxide in formation water present in an underground brine aquifer.
An example of such a method is as follows: carbon dioxide is dispersed in the form of microbubbles in water or seawater, and the resultant water or seawater is dissolved in the sea, thereby disposing of microparticles of carbon dioxide hydrate on the bottom of the ocean (Patent Document 1).
According to another method, formation water is pumped up from an aquifer; carbon dioxide is injected into the water in the form of microbubbles; and the resultant gas-liquid mixed fluid is injected into the aquifer (Patent Document 2, Patent Document 3).